In a fuel cell, reactant gas generates by galvanical reactions electricity with high efficiency. The reactant gas, such as hydrogen and carbon monoxide, entering the anode of the cell is usually derived from hydrocarboneous sources like an alcohol or fossil fuel by steam reforming processes. The fuel has to be converted to reactant gas over a reforming catalyst in the anode compartment of the fuel cell prior to contacting the reactant gas with the anode. Waste heat from the exothermic electrochemical reactions in the fuel cell is thereby used to the endothermic reforming reactions.
In conventionally designed fuel cell systems comprising e.g. an internal reforming fuel cell, which operates at high temperatures, the incoming gas is equilibrated in the internal inlet reforming unit at a temperature and pressure close to the operating conditions of the cell, usually at 600.degree.-700.degree. C. and a pressure slightly higher than atmospheric, depending on the mechanical conditions of the cell. Hydrocarbons are thereby converted to the anode reactant gas by the following reactions: ##STR1##
In the anode chamber of a molten carbonate fuel cell, about 75% of the anode reactant gas containing hydrogen, carbon oxides and unconverted lower hydrocarbons is electrochemically reacted with an oxidant; thereby one mole water and one mole carbon dioxide is added to the anode exhaust gas for each mole hydrogen converted in the cell, due to electron transport from the cathode to the anode by means of the CO.sub.3.sup.2- ions.
This results in an anode exhaust gas containing unconverted hydrogen, carbon monoxide, carbon dioxide and lower hydrocarbons which are present in the anode fuel reactant gas. At about 650.degree. C., which is the working temperature of the molten carbonate fuel cell, the anode material, usually nickel, has catalytic activity to the shift reaction (2), and a strongly reduced activity to the reforming reaction (1). A reforming catalyst arranged in direct contant with the gas surrounding the anode will be poisoned by the electrolyte of the fuel cell. Thus, the content of lower hydrocarbons in the equilibrated anode reactant gas is not available to the electrochemical process.
Unconverted hydrogen in the exhaust gas of the anode chamber is usually recovered and recycled to the anode. This is advantageously obtained in a phosphoric acid electrochemical cell as mentioned in U.S. Pat. No. 4,532,192 or by the known absorption and diffusion methods, such as molecular sieves absorption or palladium membrane diffusion (cfr "Ullmanns Encyklopadie der technischen Chemie", 3rd edition, Vol. 18, pp. 521-522 and 527-528). By the known methods about 90% of hydrogen is recovered. However, lower hydrocarbons are not recovered by these methods and thus lost with respect to electricity generation.